Bisimide resins, e.g. bismaleimide resins, are advantageously used in providing resin matrix composites, e.g. glass or carbon fiber reinforced laminates, to achieve enhanced properties such as greater thermal stability and lower moisture sensitivity than is possible with other composites, e.g. composites based on epoxy or other resins. A common bisimide, i.e. bis(4-maleimidophenyl)methane, exhibits a cure temperature close to its melt temperature. Its use in commercial manufacture of laminates is facilitated by melt point depression through the addition of methylene dianiline (which exhibits undesirable toxicity).
Nishikawa, et al., disclose in U.S. Pat. No. 4,460,783 certain aromatic ether bismaleimide compounds such as bis(maleimidophenoxyphenyl) propane and the like. Nishikawa, et al., disclose that such ether imides are highly soluble in desirable solvents such as acetone, toluene, methyl ethyl ketone and the like. See also Harvey et al in "New Aromatic-Ether Bismaleimide Matrix Resins", ANTEC '86, page 1311.
It has been discovered that the solubility of such aromatic ether bismaleimides is dependent on the presence of a considerable amount of acid group-containing precursor, i.e. having terminal amic acid groups not converted to the terminal imide group. Such acidic precursor material advantageously renders the bisimide soluble. However the terminal acid groups will tend to liberate water from ring closing imidization during cure of such resin. Such water will be vaporized during normal curing conditions and may tend to generate voids or blisters in fabricated articles such as laminates. Such water liberation is tolerable in some fabrication practices that can accommodate liberated water. However, in other applications it is especially desirable that bisimide resins cure with minimal liberation of water. An object of this invention is to provide such aromatic ether bismaleimide resin comprising substantially low levels of imide precursor acid groups.